Achievement in Judo is recognized by a series of ranks. The student ranks are called Kyu and are usually differentiated by colored belts, although different colors may be used around the world. The ten black belt, or expert, ranks are called dans.
In the days before Jigoro Kano created Judo, there was no kyu/dan rank system in the martial arts. A more traditional method of recognizing achievement was the presentation of scrolls, often with the secrets of the school inscribed. Dr. Kano started the modern rank system when he awarded shodan (first degree black belt) to two of his senior students in 1883. Even then, there was no exteernal differentiation between yudansha (black belt ranks) and mudansha (those who had not yet obtained black belt rank). Dr. Kano apparently began the custom of having his yudansha wear black obi (belts) in 1886. These obi were not the belts karateka and judoka wear today, Dr. Kano had not invented the judo gi yet, and his students were still practicing in kimono. They were the wide obi still worn with the formal kimono. In 1907, Dr. Kano introduced the modern judogi and its modern obi, but he still only used white and black belt ranks. The other colored belts originated later when Judo began being practiced outside of Japan.
The judo practice uniform and belt system eventually spread to many other modern martial arts which adapted them for their purpose. Karateka in Okinawa did not use any sort of special uniform at all in the old days. The kyu/dan ranking system, and modern karategi were first adopted by Funakoshi in an effort to encourage karate's acceptance by the Japanese. He awarded the first shodan ranks in karate on April 10, 1924. The adoption of the kyu/dan system and the adoption of a standard uniform based on the judogi were 2 of the 4 conditions which the Dai-Nippon Butokukai required before recognizing karate as a "real" martial art. If you look at photographs of Okinawa karateka training in the early part of the century, you will see that they were training in their everyday clothes.
